Two Methods of the Blow Molding Process

What is Blow Molding?

Blow molding is a process in manufacturing that creates hollow or shaped thermoplastic parts from a melting pot of polymer. The processes always begin with the polymer being melted down so that the machine can pull from the vat and inject the polymer into molds. These molds will allow the polymer to cool enough for it to be removed as a finished part, rather than requiring more machining to make it match any specifications requested. This process is used in all industries from equipment parts to appliances and toys.

Extrusion Blow Molding

Extrusion blow molding is one method that is used to create products. The process is as follows:

The Melting Stage:

The polymer is put into a melting pot that liquifies the thermoplastic down to a form that is workable and can be injected into a mold without it becoming solid in the process.

Extrusion Stage:

This is where this process is unique to others, the melting vat extrudes the polymer using a screw style piston into a hollow tube within the machine. This tube is called a parison, and the parison is then closed off from the rest of the hot material into a cool mold. The mold is either the finished part, or in the case of a hollow product, a rod pushes air into the semi-melted polymer. The air within the center of the part pushes the outer bounds of the polymer into its final shape.

Cooling and Removal:

The mold is left alone until the finish product has cooled sufficiently enough to where removal will not damage the part. The mold is then opened, and the final part is removed and ready to be shipped.

Variations in Extrusion Blow Molding

Continuous extrusion blow molding is the process where the machine is run nonstop, and the melt is continuously pushed through where the individual parts are typically removed manually with a knife sharp enough to perfectly cut the parts.

Intermittent blow molding has two different processes. The first of which is called straight intermittent. In straight intermittent extrusion blow molding, the process is very similar to injection molding. This means that the screw turns enough to mold each part, then only continues after the process of each part is completed. The accumulator method gathers melted polymer, and when the last run has cooled a rod will apply pressure, forcing the new polymer into the mold. This process makes manufacturing faster. However, a continuous accumulator process can cause difficulties when trying to get the thickness of the walls correct on hollow parts.

Injection Blow Molding

Injection blow molding is a process that allows for the fast manufacturing of polymer parts. It also allows for the manufacturing of large quantities of parts. The process is as follows:

The Melting Stage:

Like the other forms of blow molding, the polymers are melted in a vat to be blow molded into the finished parts.

The Injection Stage:

The vat is then injected into the mold of the final part it will be when the process is completed. The injection process consists of the melted polymer being pushed through a needle or injector into the mold. This can either be a needle style mold, or a solid mold depending on whether or not the part will be hollow.

During this stage, the polymer is injected into the mold. If the part is to be hollow, a needle style mold is used so that air can be injected into the center of the mold, pushing the polymers against the outside of the mold. If the part is a solid part, then the part is just injected into the mold.

Cooling and Removal:

This is the final stage where the mold is cooled enough so that the polymer will be rigid enough for removal without damaging the final product. Once it is cooled, it is removed usually by automated sections of the machine and is ready to move onto the next part in the overall process.

Injection Blow Molding in Manufacturing

In the injection blow molding process, the machine is based off of the extrusion process where a screw style piston pushes melted polymer towards the rest of the machine. The piston pushes the polymer into a hot runner manifold. This keeps the polymer in its liquid state and helps keep the material from solidifying during the injection blow molding process. After the material exits the manifold, it goes into hoses that feed into nozzles that inject the polymer into the heated molds.

If the finished part is hollow, then it also contains a core pin. A solid part that is not hollow is injected into the mold. The mold is then chilled and the polymer is solid. This solid polymer is the final product and is rotated to the ejection position where it is pushed out with other finished parts. If the part is hollow, then the material is pushed into a heated mold that contains a core pin. The core pin allows for compressed air to be injected into the mold, pushing the material against the walls of the mold, giving the part its final shape. The mold is then cooled and the part ejected with the other finished parts.

Why Use Injection Molded Parts?

Blow molded parts have been enhanced so much that they will match OEM specifications of metal parts, even though they are thermoplastic polymers. The process of injection blow molding has only increased the speed of which the parts can be created, while keeping the cost of the part low.

The parts are made to exact specifications thanks to the special molds that are used during the processes. This also helps reduce any human error that can arise from the manufacturing of these parts by making the parts exactly the same.

These methods are used by a number of manufacturers that are trying to keep the cost that their customers have to pay low, without losing quality in the products.

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